Split the find_dependency macro into a separate file.

This allows Config file authors to use it without having to use
CONFIGURE_PACKAGE_CONFIG_FILE.
Help: Add a CMake manual for Packages related docs.
Merge topic 'cmake-packages-manual' into next

9b36c7b Help: Add a CMake manual for Packages related docs.
9dcc295 Split the find_dependency macro into a separate file.